The invention relates to the field of micropropulsion systems and thrusters. More particularly, the invention relates to a microthruster having an encapsulated combustible fuel cell and an exhaust plenum for collecting debris during combustion of the fuel cell.
Propulsion systems have long been used in spacecraft. U.S. Pat. No. 4,840,024 issued Jun. 6, 1989 discloses a propulsion system, U.S. Pat. No. 5,924,278 issued Jul. 7, 1999 discloses a insulating nozzle thruster system, U.S. Pat. No. 4,318,028 discloses a gas nozzle thruster system. The advent of miniature satellites depends on the development of suitable micropropulsion systems. Solid propellant digital thrusters are currently under development. A pyrotechnic fuel charge is set off by a circuit at the bottom surface of the fuel charge. Solid propellants require elevated pressure for efficient and complete combustion. If such pressure is not built up and sustained during the burn, the resultant thrust is erratic, fragments of propellant as well as burst diaphragm are expelled randomly outward. Conventional solid rocket motors are designed to create a pressure increase upon ignition, and to expand the pressurized combustion gases through a converging diverging nozzle, to efficiently extract thrust from the propellant.
Microthruster propulsion systems have used combustible fuel cells that are selectively fired for propelling a spacecraft. The microthruster has an exhaust aligned in a single firing line directly from the fuel cells. The combustible fuel cells are covered with a fuel cap that is exploded outward thus ejecting uncontrolled debris in a single exhaust line directly away from the microthruster and the spacecraft. One problem associated with microthrusters is the exhaust of debris that provides variable microthrusting force vectors leading to an inability to accurately predict the force vector applied when firing a fuel cell. Another problem with the microthruster is the that the expelled debris may collect on sensitive spacecraft monitoring equipment and solar power collection systems thus disadvantageously contaminating spacecraft systems during a space mission. Previous designs of impulsive microthrusters include a reduced thickness section that bursts upon ignition of the charge or fuel cell by an ignition circuit. The ignition circuit is placed on the surface of the cell in an array of microthrusters each one individually addressable. The expulsion of propellant diaphragm debris fragments is caused by the back pressure at the ignition surface. The microthruster lacks a combustion chamber and can not provide increase controlled pressure for complete combustion. Consequently, the propellant is not completely burned, the burn conditions are variable and uncontrolled, and the debris and thrust vectors are erratic. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a microthruster with a clean exhaust.
Another object of the invention is to provide a microthruster with a predictable exhaust force vector when firing a combustible fuel cell.
Yet another object of the invention is to provide a microthruster with a combustible fuel in combination with a debris plenum for collecting debris remains during firing of the combustible cell for providing a clean exhaust and a predictable exhaust force vector.
A further object of the invention is to provide a microthruster with increase thrusting force by increasing combustion pressure.
Still a further object of the invention is to provide a microthruster with increased thrusting forces by increasing combustion pressure using a bent exhaust system.
The invention is directed to a microthruster having a combustible fuel cell coupled to a debris plenum surrounding an exhaust stream for collecting debris during firing of the combustible fuel so as to reduce the debris from the exhaust. The fuel cell is fired from an inverted side having a burst diaphragm that serves to encapsulate and ignite the combustible fuel cell. Upon firing, the diaphragm is exploded ejecting debris in an inverted direction and through a plenum serving to collect the debris. The exhaust is communicated through the exhaust system, including the plenum, serving to further increase the pressure within the combusting cell for increased exhaust thrust.
The solid propellant microthruster includes a propellant charge or fuel cell that is ignited by an ignition circuit. This ignition leads to the reduced likelihood of expelling propellant fragments, and improved burn area characteristics for more efficient combustion. The ignition circuit is mounted on a thin diaphragm that bursts upon ignition. The burst fragments are trapped in the plenum that allows combustion to progress and pressure to build up. The exhaust flow becomes lateral through one or more lateral ports and then the exhaust flow enters the converging diverging nozzle, and then the exhaust is finally expelled at the top surface of the microthruster with an appropriate expansion ratio and well defined thrust vector. A preferred folded flow design traps physical fragments of the burst diaphragm. A micromachined converging diverging micronozzle is achieved by specialized microprocessing to efficiently convert the pressure to flow and thrust. The exhaust flow path prevents expulsion of burst diaphragm fragments and flow is preferably passed through a micronozzle having a converging diverging profile for improved thrust efficiency. The thrust impulse is consistent in magnitude and direction and efficiently converts propellant energy to thrust.
The microthruster is suitable for replication during mass production for cost reduction. The microthruster design provides for ignition on a downstream surface of the propellant grain as in conventional solid motors. The microthruster controls the combustion pressure and achieves efficient combustion while being suitable for micromachining processes and materials during manufacturing. The microthruster is suitable for spacecraft microthrusting applications and for compact miniature gas generation systems for new generation inflatable space structures. The microthruster that can be replicated by mass production and micromachining methods with broad application in a wide variety of miniature spacecraft. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.